Cellulosic products and methods for making such products are well known in the art. Cellulosic products are typically made by draining a fiber-containing suspension and forming a web on a wire. The suspension is usually contained in a headbox before being deposited as a thin layer on the wire. The fiber web is typically dewatered by vacuum dewatering and pressing operations wherein the web is subjected to pressure developed by opposing mechanical members, for example cylindrical rolls or an extended nip press.
Cellulosic products generally have a low wet strength and wet stiffness and often exhibit dimensional changes in humid conditions which can limit their performance and usefulness. Therefore, dimensional stability is an important factor, for instance in packaging materials. However, it has been difficult to increase the wet strength without simultaneously increasing the dry strength to the same extent. When the dry strength increases too much, cellulosic products, such as cardboard and tissue paper, may become too brittle or too hard when dry which is undesirable in many applications. It is desirable that tissue paper is strong when wet but soft when in a dry state. Cardboard should have good dimension stability when wet or damp but it should not be too brittle when dry. Therefore, it would be desirable to increase the wet strength and/or the wet stiffness without substantially influencing the dry strength and dry stiffness so as to increase the so called relative wet strength (RWStr) and relative wet stiffness (RWSti).
In the prior art, there have been several attempts to improve the wet strength and wet stiffness of cellulosic products.
U.S. Pat. No. 2,980,558 discloses a process in which a paper corrugating medium is impregnated with an essentially salt-free sol of active, non-aggregated silica at a pH below 6.0 to improve the stiffness of the corrugating medium at high relative humidity.
U.S. Pat. No. 4,033,913 discloses a process in which cellulose fibers are impregnated with a solution of monomer-oligomeric silicic acid to increase dry strength, wet strength, stiffness and chemical resistance of papers for technical use, such as filters for corrosive and oxidative liquids.
However, there is still a need to improve the wet strength and/or the wet stiffness properties of cellulosic products. It is an object of the present invention to provide a process which improves at least one of the parameters wet strength, wet stiffness, relative wet strength and/or relative wet stiffness of cellulosic products.
Another object of the present invention is to provide a dispersion imparting improved wet strength, wet stiffness, relative wet strength and/or relative wet stiffness to cellulosic products. Particularly, it is an object to provide a dispersion comprising environmentally adapted products, such as substantially aldehyde-free wet strength agents.